In a cottage in Llanfairpwll on Anglesey, a group of local women gathered on September 16 1915 to discuss their contribution to the war effort. Over cups of tea and home-made cake, the 20-strong band came up with a plan: they would grow vegetables, make preserves and sew handicrafts for soldiers and their families across Britain. Their activities were the beginnings of the Women’s Institute, a body which, in just the next three years, would expand to 199 groups and play a vital role in boosting morale on the Home Front.

If those founding members could see the WI today, they might be in for a shock. Ninety-eight years later, their movement numbers 210,000 participants across 6,600 clubs in England and Wales. No longer beset by the “jam-and-Jerusalem” cliché acquired in the Fifties, the WI is attracting women of all ages and from all backgrounds, from trendy city dwellers to countryside traditionalists.

Still, even its most ardent supporters were surprised this week when pictures emerged of 350 women queuing outside a church hall in Bromley Heath, Bristol, at the opening of a new WI branch. Many were said to have been inspired by the recent series of The Great British Bake Off. “It was phenomenal,” says Sam Burch, the local president. “We knew there was a lot of interest but I never expected that many to turn up.”

And yet, just five years ago, membership of the WI was at its lowest in decades. Having peaked in 1954 with 467,000 women, its ageing leadership and stuffy reputation had led to members deserting in their droves and a dearth of new recruits. So what’s behind the resurgence of the WI – and has it, at nearly a century old, really reinvented itself?

Inevitably, the organisation has changed over the years. Church notice boards have been replaced by Twitter, crocheting by “sexy sewing” and tea dances by burlesque sessions, curry-tasting and risqué calendars. Weekend outings now encompass white‑water rafting and gin-making; speakers include forensic scientists and female Beefeaters; even their anthem Jerusalem, played at the start of each meeting, can be downloaded on iTunes.

Most significant, however, is the rise of the urban WIs. Staying true to their roots, WI groups had previously sprung up in rural communities as a way of bringing locals together. That changed 10 years ago, when young professionals wanting to meet like-minded women set up their own WI in Fulham, west London. Interest ballooned, with branches now in universities, prisons, and even in themed form, such as the gothic Iron Maidens in Merseyside.

Among the first of the city clubs was the N1 WI in north London, where members range between 25 and 55 years old. “There’s a trend for getting to know people who live near you. I think it’s a reaction to living in a busy capital city,” says one, 35‑year-old Amy Scrivener. “I have always been a joiner; I was in the Brownies and this was right up my street.”

But if the anonymity of city living has added to the WI’s appeal, it is no coincidence that its revival began in 2008, as Britain entered its worst economic slump in generations. Gripped by a need to make-do-and-mend, the nation has resorted to the comforting, nostalgic pursuits of old: baking, knitting and crafts. “After the credit crunch, people started cooking at home, buying their clothes at charity shops, preserving food, upcycling furniture – all activities that people associate with the WI,” says Amy Cotterill, 31, of the WI Girls (WIGs) based in Colchester, Essex. “The world has changed since the WI was set up. Nowadays, people move around a lot, they change jobs – it is a fragmented, broken society. Joining the WI is a way of grounding yourself.”

Celebrity endorsement, too, has helped the WI shed its staid image. Kirstie Allsopp, the television presenter, joined a west London branch in 2011 (though she said she felt like “an elephant in high heels”), and the racy exploits of the Rylstone and District set – who stripped off and posed with strategically placed buns – gave rise to a successful film, Calendar Girls, starring Helen Mirren, in 2003.

The WI’s annual campaigns also win headlines, with recent causes including calls for more midwives, ending human trafficking and saving bees. The organisation does not shy away from controversy. Janice Langley, chair of the WI National Federation, attended the national conference held at Wembley in June 2000, when Tony Blair, then prime minister, was heckled and slow-clapped during his speech. “It was unbelievable; quite the experience,” she recalls. “It was women showing how they feel about things and I felt very proud.”

With success, however, comes discord, and some members say the new band of pole-dancing, sushi-rolling WIs are at odds with the society’s roots. The Shoreditch Sisters, a feminist WI in east London, are one of many branches who refuse to sing Jerusalem. “Some of the things we want to talk about can be quite 'out there’, so we have to tailor them to cater for older members,” says treasurer Julia Davis, 26.

Still, the WI did not survive a century of modernisation by being inflexible. “It’s great to see young people joining; it shows we’re not all 97 and sitting around drinking tea,” says Langley. “The older members are quite happy to watch the belly dancing. Some want to join in. We’ve encouraged change, but some members are also happy doing what they’ve been doing since the start.”

Though the modern WI may look altered, much, she notes, remains the same. “We’ve been making buns for decades and now they’re called 'cupcakes’. Just because something is fancier on the outside, doesn’t mean it’s different on the inside.”

Perhaps those founding members would be impressed by their legacy – but not quite so surprised, after all.